5.3.4.1 The Biotite Zone

We have already given above a description of the porphyroblastic growth of the biotites associated with the SW vergent Nyimaling Tsarap nappe (D2-D3) for the regions south and west of the Gumburanjun mountain. In the area between the northern end of the Gumburanjun and Chankar, extensional movements (D4) associated with the ZSZ are clearly evidenced by C'-type shear band cleavages (Fig 5.9). At the top of this mylonitic zone, the biotite poikiloblasts form mica fishes moulded by S4. Further down, and as ductile deformation becomes more intense, biotite forms flakes that define the main fabric associated with extensional shearing.

High angle normal brittle faults (Fig 5.10) and pseudotachylites (Fig 5.11) associated with the last episode of exhumation (D6) are frequently observed within the biotite zone. These late movements shatter the biotite blasts which are then often pseudomorphed by chlorite (retrograde pseudomorphism).